Energy consumption has always been and still is an important issue for mobile terminals of wireless communication systems such as cellular systems. Recently, trends in environmental friendly technology make it important to reduce the energy consumption also for the network nodes (e.g. base stations—BS, NodeB, eNodeB, etc). Unlike terminals, which have possibilities to go into idle (or sleep) mode once they are not active, a network node needs to always transmit some signals (e.g. pilots, synchronization symbols and broadcast messages) so that camping terminals may stay in synchronization (in sync) with the network.
In an example scenario, when a network node serves a cell with many active terminals and some idle terminals, the network node may need to continuously transmit pilots, synchronization signals, etc, in order to maintain connection to the terminals. However, in an alternative scenario, when a network node serves a cell where all terminals are in an idle state (or in an active state with very low activity), the situation is somewhat different. In such scenarios, the terminals do not receive any data (or only a very small amount of data). The terminals only need to do mobility measurements (such as measurements for cell reselection or handover) and listen to paging messages. In such scenarios, a network node which is transmitting according to its conventional transmission scheme will waste a lot of energy.
Proposals considering an ability to put network nodes with a low load (e.g. serving no or few active terminals) into an “idle” state have been discussed.
An “idle” state (or energy saving mode) of a network node may be applicable when there are no active terminals in a cell associated with the network node. Alternatively or additionally, the “idle” state of a network node may be applicable when the number of active terminals in the cell are less than or equal to a threshold.
In the same manner a normal operation mode of a network node may be applicable when there is at least one active terminal in a cell associated with the network node. Alternatively or additionally, the normal operation mode may be applicable when the number of active terminals in the cell is greater than a threshold (which may or may not be the same as the threshold for the “idle” state).
Generally, the “idle” state may comprise restricted downlink transmissions. For example, some signals may be transmitted intermittently instead of continuously, and/or some signals may be transmitted using only part of the cell transmission bandwidth, and/or some signals may not be transmitted at all.
An example of a network node “idle” state may be a low activity mode of the network node comprising restricted or reduced downlink transmissions. One possible approach might be to reduce the amount of transmitted signals or channels to an absolute minimum. There are numerous possible ways to achieve a low activity mode of a network node, and the possibilities may vary depending on the access technology under consideration. Definitions and particulars of a network node “idle” state may be specified in standardization documentation for the applicable standards. Low activity state, low transmission state, restricted state, and restricted transmission state are other examples of possible terminologies that may be used for the idle state. The skilled person recognizes that all of these terms may have similar or the same meaning. Hereinafter, the terms idle state, energy saving mode and low activity mode will be used. A few examples of approaches that are applicable to UMTS LTE (Universal Mobile Telecommunication Standard Long Term Evolution, Release 9, 10 and onwards; E-UTRAN—Evolved UMTS Terrestrial Radio Access Network) are given in the following. It should be noted however, that these are merely examples and by no way limiting to the invention.
In UMTS LTE it may be sufficient for a NodeB in the “idle” state to transmit only synchronization signals (PSS—Primary Synchronization Sequence/SSS—Secondary Synchronization Sequence) and physical broadcast information (P-BCH—Physical Broadcast CHannel) together with pilot symbols (also denoted reference signals and used for e.g. channel estimation) in association with the P-BCH transmission. These signals are transmitted in well defined OFDM symbols in sub-frames 0 and 5 in both FDD (Frequency Division Duplex) and TDD (Time Division Duplex). If such an approach is used, the NodeB can reduce its duty cycle significantly and basically transmit information only in a few OFDM symbols of the sub-frames 0 and 5.
If more limited pilot (or reference) signals are transmitted in the energy saving mode (e.g. only in sub-frames 0 and 5, and only over 6 central resource blocks, then a terminal may perform measurements over a smaller bandwidth than when the network node is in non-energy saving mode.
In another approach that further reduces the amount of transmissions in UMTS LTE, only the synchronization signals (PSS/SSS) are transmitted by the network node when it is in the “idle” state. As in the previous example above these signals are transmitted in sub-frames 0 and 5. The transmission takes place over 6 central resource blocks regardless of the actual cell transmission bandwidth (i.e. the bandwidth in the active state).
A problem that arises when reducing the number of pilots is that the terminal needs to rely on fewer symbols for keeping in sync with the network node.
Another problem that arises is that the operations of a terminal (e.g. measurements, paging listening, etc) designed to work well when the corresponding network node is in a normal operation mode may not be optimal when the corresponding network node is in the “idle” state.
Thus, there is a need for methods and arrangements for enabling a terminal to operate adequately in a wireless communication network having network nodes with two operational modes, e.g. a high activity mode (conventional operation) and a low activity mode (“idle” state).